The human cytomegalovirus terminase complex as an antiviral target: a close-up view

Abstract

Human cytomegalovirus (HCMV) is responsible for life-threatening infections in immunocompromised individuals and can cause serious congenital malformations. Available antivirals target the viral polymerase but are subject to cross-resistance and toxicity. New antivirals targeting other replication steps and inducing fewer adverse effects are therefore needed. During HCMV replication, DNA maturation and packaging are performed by the terminase complex, which cleaves DNA to package the genome into the capsid. Identified in herpesviruses and bacteriophages, and with no counterpart in mammalian cells, these terminase proteins are ideal targets for highly specific antivirals. A new terminase inhibitor, letermovir, recently proved effective against HCMV in phase III clinical trials, but the mechanism of action is unclear. Letermovir has no significant activity against other herpesvirus or non-human CMV. This review focuses on the highly conserved mechanism of HCMV DNA-packaging and the potential of the terminase complex to serve as an antiviral target. We describe the intrinsic mechanism of DNA-packaging, highlighting the structure-function relationship of HCMV terminase complex components.

Figure 1.

From full genome to cleavage site. Rolling circle replication results in the formation of head-to-tail concatamers that further act as substrates for the DNA-packaging process. The genome is organised as two regions. The unique long (UL) and the unique short (US) segments are flanked by repeated sequences that contain the ≪ a ≫ sequence. The pac1 and pac2 sequences are present in each ≪ a ≫ sequence.

Figure 2.

Genome cleavage/packaging and the HCMV terminase complexe adapted from Bogner, Radsak and Stinski (1998). (i) Translocation of the terminase complex into the nucleus, (ii) HCMV terminase specifically binds the pac site and recruits the empty capsid, (iii) cleaves the duplex, (iv) exerts its ATPase activity to power translocation of a unit-length DNA genome into the capsid and (v) completes the DNA-packaging process by cutting off excess DNA at the portal region. (vi) Finally, the DNA-terminase complex dissociates from the filled capsid and is ready for next DNA-packaging step.

Figure 3.

Terminase subunit pUL56 conserved regions adapted from Champier et al. (2008). pUL56 is composed of 12 conserved regions (I-XII). The conserved region IV represents the pUL56 zinc-finger domain. The central region of pUL56 and the C-terminus include two variable regions annotated VRI and VRII. The three putative leucine zippers, annotated pUL56-LZ, are indicated. The short sequence in the C-terminal region of pUL56 (₆₇₁WMVVKYMGFF₆₈₀), essential for interaction with pUL89 is highlighted. Positions of amino acids associated with in vitro resistance to letermovir are highlighted. Resistance mutations that have been identified in clinical studies are notified in bold with a star. The position of the Q204R benzimidazole resistance mutation is shown in red.

Figure 4.

Terminase subunit pUL89 conserved regions adapted from Champier et al. (2007). pUL89 is composed of 12 conserved regions (I-XII) with several putative functional domains such as nuclear localisation site (NLS-1 and NLS-2), pUL89 zinc-finger domain annotated pUL89-ZF, adenine binding site (Walker A box, Walker B box and ATPase coupling helicase, ATP binding site. Underlined amino acids are residues involved in the activities of domains. Positions of amino acids associated with in vitro resistance to letermovir are highlighted. The position of the D344E and A355T benzimidazole resistance mutations are shown in red.